Mayella Ewell dominates Chapter 18, when she testifies against Tom Robinson. In this chapter many aspects of her personality appear. First, when she sits down in the witness chair, she starts breaking out into tears, showing she has a fragile personality. When she later starts talking about the incident with her and Tom, it is evident that she is lying and when she wants to reveal the truth, she is afraid of her father, Bob Ewell, who is evidently physically abusing her. These incidents help us to feel sympathy towards her.
During Tom Robinson's tesimony in Chapter 19, Mayella is characterized as being lonely, even more so than Boo Radley. Because she lived among filthy surroundings next to the town dump, the whites of Maycomb refused to associate with her nor would any black person because she was white. When Atticus interviewed her about who her friends were, she did not know what he meant, then assumed she was being made fun of once he explained himself. Tom Robinson is the only person who is polite to her despite the fact that they are of different races. One of Mayella's main motivators for attempting to seduce Tom Robinson is her loneliness.
Overall, Mayella is a very hated character and the reader naturally doesn't believe her evidence. Tom Robinson, on the other hand, is portrayed as an innocent character, and the reader is most likely on his side during the trial.
I was sittin' on the porch, and he come along. Uh, there's this old chifforobe in the yard, and I-I said, 'You come in here, boy, and bust up this chifforobe, and I'll give you a nickel.' So he-he come on in the yard and I go in the house to get him the nickel and I turn around, and 'fore I know it, he's on me, and I fought and hollered, but he had me around the neck, and he hit me again and again, and the next thing I knew, Papa was in the room, a-standin' over me, hollerin', 'Who done it, who done it?
-Mayella Ewell, testifying against Tom Robinson
'I got somethin' to say. And then I ain't gonna say no more. He took advantage of me. An' if you fine, fancy gentlemen ain't gonna do nothin' about it, then you're just a bunch of lousy, yella, stinkin' cowards, the - the whole bunch of ya, and your fancy airs don't come to nothin'.